1. Field of the Invention
The present invention relates to an image forming apparatus and method, and more particularly, to the structure of a recording head unit suitable for an inkjet recording apparatus forming color images by using inks of a plurality of colors, and to a recording control technology for same.
2. Description of the Related Art
When printing a color image, an inkjet printer uses inks of at least three colors, cyan (C), magenta (M), and yellow (Y), and furthermore, it may also form images using black (Bk), light cyan (LC), light magenta (LM), dark yellow (DY) and a special color (SPC), and the like.
In general, in a printer for producing print outputs of high quality (photographic quality), inks of six or more colors, including the addition of the light cyan (LC) and light magenta (LM) described above, are often used in order that the contrast between the grains of the printed dots is not noticeable. In inkjet printers of this kind, generally, the nozzle density in the head is set to the same density for each of the colors. Examples are known wherein document printing speed is emphasized, and Bk nozzles only are provided in greater number and higher density than the other colors, but in this case, all of the colors other than Bk are set to the same nozzle density as each other. More specifically, in general, if the number of colors is increased due to demands for high quality, then the number of nozzles also increases, accordingly.
Furthermore, in a conventional inkjet printer, in order to shorten the printing time, the time interval between ink discharges has been shortened (the discharge frequency has been increased), and the number of ink discharge nozzles in the recording head has been increased. Increase in the discharge frequency has been achieved either by raising the upper limit of the response frequency of the discharge mechanism (the pressurizing devices, such as a piezo element, or the heater), or by replenishing ink more quickly after ink discharge, or the like. Furthermore, increasing the number of discharge nozzles is achieved by improving the head processing and fabrication technology, and increasing miniaturization and density, and even in an inexpensive inkjet printer, the overall number of nozzles can be several thousand.
More specifically, the trend of technological development is moving towards heads of ever larger overall size, due to the multiplying effect of the number of colors and the number of nozzles in response to demands for high quality and speed.
Due to improvements of these kinds, it has been possible to shorten the printing time, but on the other hand, the following types of problems have arisen. More specifically, the increase in the number of nozzles described above leads to problems in that, in addition to raising the cost of the device, the increase in the total number of nozzles, and the fact that the total length of the flow passages inside the head for supplying ink to these respective nozzles becomes longer, give rise to an increased possibility of an ink discharge problem occurring in the head.
This is not limited to an increased probability of simple breakdowns, but rather means that there is an increased possibility of problems such as air bubbles becoming trapped inside the ink flow passages and it becoming impossible to perform normal discharge, or problems which are intrinsic to inkjet systems, such as the ink viscosity rising in the vicinity of the nozzles, and causing discharge failures.
More particularly, in a single pulse type inkjet printer, which, unlike a shuttle scan type printer for printing by scanning an inkjet head back and forth, has a fixed head of a length equal to or greater than the print image and performs printing by conveying printing paper in a direction orthogonal to the longitudinal direction of the head, the number of nozzles per ink color may exceed 10,000, and hence the issue of increased possibility of problems such as those described above is very serious indeed.
Furthermore, if inks of six colors are used in a single pass type inkjet printer, then naturally, the overall size of the head will become very large, and the cost thereof will increase.
In order to deal with the issue of problems of this kind, although it runs counter to improvements aimed at enhancing image quality, if the number of nozzles could be reduced, then the possibility of problems occurring can also be reduced.
In relation to technology for improving image quality in an inkjet recording apparatus, Japanese Patent Application Publication No. 9-286125 proposes a method for recording respective inks at a level of resolution that corresponds to the color appearance. The object of Japanese Patent Application Publication No. 9-286125, as is evident from the statement that “recording is carried out at an image resolution corresponding to the color appearance, for each ink, independently”, is to achieve the minimum required image resolution and to reduce the amount of image sent to the printer. Therefore, one pixel is recorded either by ejecting a plurality of droplets of dilute ink, or by means of a plurality of ink dots. However, ejecting droplets at different resolutions in this way is extremely complex, when it comes to carrying out image processing and determining the location of the dots. Furthermore, if a plurality of ink droplets are simply ejected, then since there is a limit on the capacity of the paper to receive ink, this is not a practicable way of achieving high image quality.
Japanese Patent Application Publication No. 10-211692 discloses technology for performing substitute recording using a low-density ink in the event of discharge failure of a high-density ink, in an apparatus having a recording head discharging inks of the same color and different densities. This technology has the object of providing a response for emergency use, in cases of an abnormality, wherein substitute recording is carried out by using a low-density ink, if there has been a discharge failure with a high-density ink, and it is similar to the disclosure in Japanese Patent Application Publication No. 9-286125, in that, if there is a blockage, or if the dark ink has run out, then a plurality of droplets of lighter ink are ejected, or a large droplet of same is ejected.
Japanese Patent Application Publication No. 10-44475 discloses technology for suppressing the volume of ink by raising the concentration of ink having high brightness or low perception of graininess, and performing correction to reducing the recorded ink volume of same, and it states yellow (Y) as the ink to which this is applied. This technology has the object of suppressing the overall ink volume by making the yellow ink darker than the ink of other colors, and controlling the amount of yellow ink used in such a manner that it is reduced, and for this purpose, it stipulates a relationship between the concentrations of the respective inks. Therefore, it does not disclose information of particularly great value with regard to reducing the number of colors (number of nozzles).